1) Armigers increase hit dice to 1d4+8 (used to be 1d4+6). People whose sole tactic relies on getting smacked deserve to be as tough as you can be.

2) Harriers increase their Defense Bonus from the Good progression to the Excellent progression. This better shows their speed and agility, and now we don't have a progression that's not used by anyone. (Yeah, think that one over.)

3) In standard IH every saving throw is equal to your class level (plus other adjustments as normal). This has the result of high level characters being nigh immune to everything requiring a save, plus a bland homogenizing effect.
Instead you may yourself pick which one of your saves is bad, which one good, and which one excellent. This is not determined by class but total character level; maybe your assassin is nimble (excellent reflex) and maybe he's a brainwashed death-cult fanatic (excellent will).
Bad progression: bonus equal one-third your level (round all frations down).
Good progression: bonus equal one-half your level (round all frations down).
Excellent progression: bonus equals level. And you round fractions up too!

4) Despite what the description says, you do not get multiple attacks of opportunity against a single target in a round, no matter how many the target technically provokes. (Ex: He dances around you, drinks a potion, and then tries to grapple you. That's three provokes, but you get only 1.)
However, the seventh level of Combat Reflexes is exactly made for this situation. Without this houserule, this use of ComRef is redundant.

5) All crossbows are simple weapons, though the hand crossbow doesn't exist. Longbows are exotic weapons. Special bows (mighty f'rex) are made only by the elves, Anuirans, and Kinashi.

6) Speaking of weapons that don't exist, there are no kama, nunchaku, dire freaking flails, adamantine swords, or any of that shit.

7) Armor is changed into thus:
1 (padded), 1d2 (leather), 1d3 (studded), 1d4 (scale), 1d6 (chain), 1d8 (brigandine), 1d10 (plate).
Light armor provides no protection against magic weapons. (They effectively are 1dx/magic.)
Medium armor reduces speed by 5'. Magic weapons have no special effect against them.
Heavy armor reduces speed by 10'. They may roll their DR twice and take the best result. Against magic weapons they roll only once.

More as I think of it.

What you need to understand about the apocalypse is that you aren't Mad Max. You're part of the skull pyramid in the background.